Method of producing an electrolytic coating on aluminum and the product thereof

ABSTRACT

Integral anti-corrosive colored coatings on articles of aluminum or aluminum alloys free of an initial anodic coating are produced by passing an alternating current (A.C.) through a sulfuric acid bath containing metal ions which yield a colored integral metal sulfide and oxide on the aluminum article.

United States Patent 91 Chakravarti et al. [4 1 Feb. 20, 1973 [54] METHOD OF PRODUCING AN [56] References Cited ELECTROLYTIC COATING ON ALUMINUM AND THE PRODUCT UNITED STATES PATENTS THEREOF 3,661,729 5/1972 Miyakawa et al ..204/35 N [75] Inventors: Diptiman Chakravarti; Jack G. FOREIGN PATENTS 0 APPLICATIONS Surendranath; Richard Duncan 69,930 1/1946 Norway ..204/58 orthanofseame'wash' 762,91! 7/1967 Canada ..204/35 N [73] Assignee: Fentron Industries, Inc., Seattle,

Wash. Primary Examiner-John H. Mack 22 Filed; 27, 7 Assistant Examiner-W. I. Solomon [21] A l N 93 461 Attorney-Seed, Berry & Dowrey Related US. Application Data [57] ABSTRACT Integral anti-corrosive colored coatings on articles of [63] fggg rsgggxj of aluminum or aluminum alloys free of an initial anodic coating are produced by passing an alternating current (A.C.) through a sulfuric acid bath containing metal 1 g. 2040555310332 ions which yield a colored integral metal sulfide and [58] Field of Search ..204/35 N, 58, 42 ox'de alummum 8 Claims, N0 Drawings METHOD OF PRODUCING AN ELECTROLYTIC COATING ON ALUMINUM AND THE PRODUCT THEREOF CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of Ser. No. 696,142, filed Jan. 8, 1968 and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of producing colored anodized coatings on aluminum and to the products produced thereby.

2. Description of the Prior Art Methods to produce color anodized surfaces on aluminum or aluminum alloy articles have employed direct current (D.C.) electrolysis between the aluminum article serving as an anode and a cathode, fabricated from carbon or other suitable material, in an acid solution such as aqueous solutions of sulfuric acid capable of yielding oxygen on electrolysis. These coatings were generally only slightly colored. Furthermore, the electrolysis had to be conducted at low temperatures on the order of to 30 F. to produce hard anodic oxide layers of satisfactory abrasion resistance.

More recently, aqueous solutions of sulfosalicylic acid and minor portions of sulfuric acid have been employed at room temperatures or above in DC. anodization processes to produce hard anodic coatings. Thus far, however, the color range of such coatings has been limited to various shades of bronze with the darkness of the color being dependent on the processing parameters such as the DC. current density, the time period of anodization, the concentration of sulfuric acid and the alloy used.

The only method by which a range of colors in colored coatings has been produced on aluminum or aluminum alloys has been to first anodize the base aluminum and then color the anodized coating by the addition of organic dyes, by various hydrolysis processes, by deposition of insoluble inorganic compounds on the anodized coatings as disclosed by Asada, or by other means to effect coloration of the pores of the anodic coating. Asada (US. Pat. No. 3,382,160; Canadian Patent No. 662,063; British Patent No. 1,022,927; and Japanese Patent No. 38-1715) discloses a process for producing colored protective coatings on aluminum articles which have been previously provided by electrolytic oxidation with a film of oxide. An alternating current is passed through an aqueous bath containing a metal salt which deposits a colored metal oxide or hydroxide in the oxidized film on the aluminum surface. The resultant coloration is not integral with the anodic coating. The processes mentioned require a number of processing steps to achieve a colored surface. For example, such processes all require at least one additional processing step to color impregnate the anodic coating over the sulfosalicylic-sulfuric acid method.

SUMMARY OF THE INVENTION By the present invention, aluminum (and by aluminum" is meant high purity aluminum, aluminum in various commercial grades, and aluminum base alloys free of an initial oxide coating) articles may be provided with a hard, durable, electrolytic coating in a variety of colors with no undue complexity arising from the number of processing steps necessary to produce the coating. In general, the aluminum article is cleaned and degreased with a suitable alkaline cleaner in the manner known to the art, electrolyzed by passing alternating current through an aqueous solution consisting essentially of an organic or inorganic acid such as chromic, sulfuric, exalic, maleic, sulfosalicylic and other acids in the presence of small quantities of metal ions which react to yield a colored coating when sealed by contact with hot water, ammonium lignosulfonate and other sealing agents. The aluminum article is employed as one electrode and carbon or other suitable material is employed as the other electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Suitable cations include silver, copper, cadmium, cobalt, iron, selenium, bismuth, tin and zinc. These are provided as cations of acid salts, preferably in a highly oxygenated form such as a sulfate oras the anions of a metallic oxy acid such as permanganate to ensure that sufficient oxygen is present to accomplish the desired reaction.

The reaction under the alternating current conditions is exceedingly complex. Considering the reaction with sulfuric acid, the preferred acid, it is theorized that the metallic ions are alternately deposited with a sulphurous film. Since hydrogen may be evolved as well as oxygen at the pore basis, the probability of very complex reduction-oxidation reactions taking place is quite reasonable, particularly since considerable pressures are produced at the pore bases. Sulfur and sulfuretted hydrogen are formed in the coating to the extent that the electrolytic coating may appear a pale yellow and smell strongly sulfurous. When the coating is sealed in hot water, the sulfur reacts with the intermetallic constituents present in the electrolytic film to form sulfides, the color of the coating thereby changing to the characteristic color of the particular metallic sulfide present. For example, the use of copper sulfate to provide the metallic constituent results in a green electrolytic coating when sealed. Cadmium sulfate produces a characteristic yellow and zinc a greyish white.

If desired, the intensity of the resulting color can be increased by pre-dipping the aluminum article in a dilute ammonia solution to reduce the solubility of the sulfate in the acid contained in the pores of the coating.

The preferred operating parameters are as follows: up to about wt. percent and preferably about 20-26 wt. percent sulfuric acid; up to a maximum of about 5.0 wt. percent and more preferably up to about 1.0 wt. percent metallic sulfate or equivalent soluble metallic compound; a voltage range of up to about 25 volts and more preferably about 9-17 volts; and electrolyte temperature of about 65-90 F.; and a sealing water temperature of 212 F. Indications are that any substantial deviation from these preferred ranges result in the production of a defective coating depending on the metallic salt selected. These ranges are relatively critical for consistently producing a satisfactory coating: for example, when copper sulfate is employed.

It is to be noted that the methods of this invention are not sensitive to multiplicity of A.C. phases. Thus, for example, the same results are achieved with both single phase and three phase alternating current, the choice being left to whichever is less costly to procure.

The following examples are illustrative of the invention and are not intended to be limiting in any manner.

Example 1 A sheet of aluminum 6063 alloy free of an initial oxide coating was immersed in an electrolytic bath of 26 percent by weight sulfuric acid to which was added 0.20 grams/liter (0.02%) copper sulfate (CuSO4-5H The aluminum sheet and a carbon electrode immersed in the bath were connected to a source of alternating current for 30 minutes at a voltage of 13 volts. The bath temperature was at 70 F. during the electrolytic oxidation. A green color was produced on the sheet of aluminum alloy on sealing in hot water. The thickness of the coating was 0.3 mil.

Example 2 A sheet of aluminum 6063 alloy free of an initial oxide coating was immersed in 26 percent by weight sulfuric acid to which was added 0.20 grams/liter of copper sulfate. The alloy and a carbon electrode also immersed in the bath were connected to a source of alternating current for 20 minutes at a voltage of 9 volts.

The bath temperature was at 70 F. during the electrolytic oxidation. A medium green sulfide coating was produced on the alloy on sealing in hot water. The thickness of the coating was 0.5 mil.

Example 3 A sheet of aluminum 6063 alloy was treated as in Example 2 except that the aluminum was subjected to the passage of alternating current for 10 minutes at a potential of 10 volts. A dark green coating, 0.9 mils in thickness was produced on sealing of the article in hot water.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. The method of forming an integral colored coating on an aluminum article not previously provided with an anodized coating, comprising:

immersing the aluminum article in an aqueous sulfuric acid electrolyte solution containing a soluble metal compound providing sufficient metal ions to yield a colored integral metal sulfide coating on the aluminum, the metal ions reacting with sulfur constituents in the coating formed on the aluminum electrolytically,

passing an alternating current through the electrolyte solution between the aluminum article and an electrode to decompose a portion of the sulfuric acid in the electrolyte solution providing sulfur constituents which are deposited together with the metal ions of the soluble metal compound on the aluminum, the metal ions reacting with the sulfur constituents in the coating to produce an integral colored coating on the aluminum.

2. The method of claim 1 wherein the metal compound is selected from the group consisting of the sulfates of silver, copper, cadmium, cobalt, iron, selenium, bismuth, tin and zinc.

3. The method dependent on claim 1 wherein the acid is provided in an amount between about 20 and 26 wt. percent; the metal sulfate is copper sulfate provided in an amount up to about 0.02 wt. percent; wherein the electrolysis takes place at a current density of between about 20 and 50 A/Ft. and a voltage between about 9 and l7 volts; and wherein the temperature of said aqueous solution is maintained at a temperature between about 65 to F.

4. The method of forming an integral colored coating on an aluminum article, the aluminum not previously provided with an anodic coating, comprising the steps of:

immersing the aluminum article in an aqueous electrolyte solution consisting essentially of sulfuric acid and a soluble metal compound of an acid salt selected from the group consisting of copper, cadmium and zinc, passing an alternating current through the electrolyte solution between the aluminum article and an electrode to decompose a portion of the sulfuric acid electrolyte, providing sulfide ions which are deposited together with metal ions of the soluble metal of the acid salt on the aluminum article, and

sealing the coating by contact with a hot sealing fluid, the metal ions in the coating reacting with the sulfide ions therein to form a metal sulfide, the color of the coating changing to the characteristic color of the metal sulfide.

5. The method of forming an integral colored coating on an aluminum article, the aluminum not previously provided with an anodic coating, comprising the steps of:

immersing the aluminum article in an aqueous solution consisting essentially of sulfuric acid and a metal sulfate selected from the group consisting of copper and cadmium sulfate,

passing an alternating current through the electrolyte solution between the aluminum article and an electrode to decompose a portion of the sulfuric acid, providing sulfide ions which are deposited together with metal ions of the metal sulfate on the aluminum article, and

sealing the aluminum article by contact with a hot sealing fluid, the sulfide ions in the coating reacting with the metal ions of the metal sulfate to form metal sulfides, the color of the aluminum article changing to the characteristic color of the particular metal sulfide on sealing.

6. An article of manufacture fabricated of aluminum wherein the aluminum has an integral colored electrolytic coating produced by the process of claim 5, the coating color achieved on sealing being characteristic of the sulfide of the metal constituent of the particular metal sulfate employed.

7. The method dependent on claim 5 wherein said acid is provided in an amount between about 4 and 26 wt. percent.

8. The method dependent on claim 5 wherein said metal sulphate is provided in an amount up to about 1 wt. percent. 

1. The method of forming an integral colored coating on an aluminum article not previously provided with an anodized coating, comprising: immersing the aluminum article in an aqueous sulfuric acid electrolyte solution containing a soluble metal compound providing sufficient metal ions to yield a colored integral metal sulfide coating on the aluminum, the metal ions reacting with sulfur constituents in the coating formed on the aluminum electrolytically, passing an alternating current through the electrolyte solution between the aluminum article and an electrode to decompose a portion of the sulfuric acid in the electrolyte solution providing sulfur constituents which are deposited together with the metal ions of the soluble metal compound on the aluminum, the metal ions reacting with the sulfur constituents in the coating to produce an integral colored coating on the aluminum.
 2. The method of claim 1 wherein the metal compound is selected from the group consisting of the sulfates of silver, copper, cadmium, cobalt, iron, selenium, bismuth, tin and zinc.
 3. The method dependent on claim 1 wherein the acid is provided in an amount between about 20 and 26 wt. percent; the metal sulfate is copper sulfate provided in an amount up to about 0.02 wt. percent; wherein the electrolysis takes place at a current density of between about 20 and 50 A/Ft.2 and a voltage between about 9 and 17 volts; and wherein the temperature of said aqueous solution is maintained at a temperature between about 65* to 90* F.
 4. The method of forming an integral colored coating on an aluminum article, the aluminum not previously provided with an anodic coating, comprising the steps of: immersing the aluminum article in an aqueous electrolyte solution consisting essentially of sulfuric acid and a soluble metal compound of an acid salt selected from the group consisting of copper, cadmium and zinc, passing an alternating current through the electrolyte solution between the aluminum article and an electrode to decompose a portion of the sulfuric acid electrolyte, providing sulfide ions which are deposited together with metal ions of the soluble metal of the acid salt on the aluminum article, and sealing the coating by contact with a hot sealing fluid, the metal ions in the coating reacting with the sulfide ions therein to form a metal sulfide, the color of the coating changing to the characteristic color of the metal sulfide.
 5. The method of forming an integral colorEd coating on an aluminum article, the aluminum not previously provided with an anodic coating, comprising the steps of: immersing the aluminum article in an aqueous solution consisting essentially of sulfuric acid and a metal sulfate selected from the group consisting of copper and cadmium sulfate, passing an alternating current through the electrolyte solution between the aluminum article and an electrode to decompose a portion of the sulfuric acid, providing sulfide ions which are deposited together with metal ions of the metal sulfate on the aluminum article, and sealing the aluminum article by contact with a hot sealing fluid, the sulfide ions in the coating reacting with the metal ions of the metal sulfate to form metal sulfides, the color of the aluminum article changing to the characteristic color of the particular metal sulfide on sealing.
 6. An article of manufacture fabricated of aluminum wherein the aluminum has an integral colored electrolytic coating produced by the process of claim 5, the coating color achieved on sealing being characteristic of the sulfide of the metal constituent of the particular metal sulfate employed.
 7. The method dependent on claim 5 wherein said acid is provided in an amount between about 4 and 26 wt. percent. 